1. Field of the Invention
This invention relates to a multi-stage catalytic process for upgrading coal pyrolysis oils with the objective of producing valuable gasoline, jet fuel, and diesel fuel. Coal Pyrolysis Oils are highly aromatic, olefinic, and are unstable. Additionally, they contain objectionable sulfur, nitrogen, and oxygen contaminants, and may also contain coal solids that would plug fixed-bed reactors.
2. Description of Prior Art
Coal is the world's most abundant fossil fuel. However, coal has three major drawbacks: (1) Coal is a solid and is less easily handled and transported than fluidic or gaseous materials; (2) Coal contains compounds which, upon burning, produce “air toxics” and the pollutants associated with acid rain; and (3) Coal is not a uniform fuel product, varying in characteristics from region to region and from mine to mine. In fossil fuels, the ratio of hydrogen atoms to carbon atoms is most important in determining the heating value per unit weight. The higher the hydrogen content, the more liquid (or gaseous) the fuel, and-the greater its heating value. Natural gas, or methane, has a hydrogen-to-carbon ratio of 4 to 1 (this is the maximum); gasoline has a ratio of about 2.2 to 1; petroleum crude about 2.0 to 1; shale oil about 1.5 to 1; and coal about 1 to 1. Thus, if coal is processed and the hydrogen on half the carbons could be transferred or “rearranged” to the other half of, the carbons, then the result would be half the carbons with 0 hydrogens and half with 2 hydrogens. The first portion of carbons (with 0 hydrogens) would be a solid char; the second portion of carbons (with 2 hydrogens) would be a liquid product similar to a petroleum derived fuel oil.
Therefore, if this could be accomplished using only the hydrogen inherent in the coal, i.e., no external hydrogen source, then the coal could be refined in the same economical manner as petroleum, yielding a slate of refined hydrocarbon products and low hydrogen containing char.
In our modern society nearly every raw material is refined prior to final use. Various raw ores are refined to produce useful products, such as aluminum, copper, silver, titanium, and tungsten. Except for coal, all of our fuels are refined: for example, uranium ore, crude oil, and natural gas are refined.
Natural gas from the wellhead contains impurities, such as CO2, heavy hydrocarbons, and sulfur containing gases. These impurities are removed prior to use to yield predominantly a single hydrocarbon compound: methane. Natural gas represents less than 3% of, the United States' known energy reserves.
Crude oil from the wellhead has limited utility. It is typically a dirty, sulfur-containing fuel. Hence, the petroleum industry has developed refining processes such as hydrocracking and fluid catalytic cracking techniques to produce value-added products, such as gasoline, jet fuel, and other hydrocarbon fuels and petrochemicals. Thus, gasoline can be produced from high sulfur crude or from light Arabian crude.
Raw coal, as it is mined, also has limited utility being used mostly for direct combustion to produce heat and steam to generate electricity. Like crude oil, coal contains complex hydrocarbons, sulfur, and nitrogen. Coal also contains large amounts of mineral matter (or ash). High sulfur bituminous coals and high moisture subbituminous coals are very different raw materials and cannot be interchanged as fuels. Coal is the most abundant fossil fuel in the U.S., accounting for over 95% of the fossil energy reserves. The United States has 43% more energy in coal reserves than the energy equivalent of all the oil and gas in known reserves in the entire world. Vast deposits of coal also exist in Eastern Europe, Russia, and China but are either far away from manufacturing regions or contain high levels of pollutants relative to the heating value of the coal.
Coal refining processes, like the petroleum refining processes, can employ a range of approaches and technologies. One coal refining approach uses a thermal treatment or pyrolysis in which a rapid heatup of the coal produces gases, a liquid product, and a char. The char can be used as a high energy content boiler fuel, the gases can be recycled and used to provide heat for the pyrolysis process, and the liquids can be refined to produce valuable transportation fuels. The coal pyrolysis oils contain objectionable sulfur, nitrogen, oxygen, and solids contaminants and are deficient in hydrogen and therefore must be further refined.
The two primary approaches for coal refining for the purpose of converting coal to liquid fuels are called direct and indirect coal liquefaction. Direct coal liquefaction (“DCL”) reacts coal in a solvent with hydrogen at high temperatures and pressure to produce liquid fuels. DCL was first developed by Dr. Bergius in Germany in 1913 and used commercially in Germany between 1927 and 1945. However, after World War II, crude oil was widely available at reasonable prices and commercial coal liquefaction was therefore not commercially attractive. As a result, very little liquid fuels sold today are produced using a coal liquefaction process.
Indirect coal liquefaction (“ICL”) involves first gasifying coal to produce a synthesis gas which contains principally carbon monoxide and hydrogen and thereafter processing the gas chemically into a variety of fuels.
Where diesel type products are desired utilizing ICL, the Fischer-Tropsch process is preferably used to convert the synthesis gas. The ICL technology was commercially applied in the 1920-1940's in Germany and since the 1950's in South Africa. While commercially demonstrated, the ICL technologies are very complex, capital intensive, and have low thermal efficiencies compared to direct coal liquefaction.
Several technologies can be utilized for the gasification step of Indirect Coal Liquefaction. Some coals are better processed in a lower temperature moving-bed gasifier (such as the Lurgi-type used by Sasol in South Africa and by the Dakota Gasification Plant in the U.S. where the desired product is synthetic natural gas). Low temperature gasification produces pyrolysis type products including tar oils, phenolics, and crude naphtha which contain objectionable sulfur, nitrogen, phenolics, olefins, and entrained solids which must be further refined to meet environmental and end use requirements. High temperature gasification such as the entrained flow gasifiers operate at high temperature conditions that completely convert the liquids formed to synthesis gases within the gasifier thus not having any direct coal liquids produced.
For coal pyrolysis oils, product quality upgrading has been demonstrated through multiple, difficult process steps. For a fixed-bed hydrotreatment approach, the oils containing entrained solids (fine coal char, ash, and slag particles) must first undergo filtration before being hydrotreated in fixed-bed reactors. The coal pyrolysis tars are unstable and viscous making filtration difficult. The fixed-bed hydrotreaters have very short catalyst cycle lengths due to catalyst bed plugging. Several catalyst beds are required with interstage hydrogen or oil quench to control the exothermic hydrogenation reaction. As peak crude oil production nears, there is a large economic incentive for these poor quality coal pyrolysis oils to be upgraded to transportation fuels.
For Direct Coal Liquefaction, extensive research and development was conducted in the 1970's and 1980's in the United States and world-wide, as oil shortages and high oil prices were experienced. The objectives were to produce transportation fuels from coal to reduce oil imports. The US Department of Energy provided financial and technical support to demonstrate two technologies on a large scale (200 ton/day coal feed). The Exxon Donor Solvent (“EDS”) technology liquefies coal with hydrogen and a hydrogen donor solvent at temperatures of 800-840° F. (427-449° C.) and pressures of 2500-3000 psia (172-207 bars). Process derived distillate coal liquids boiling at 400-700° F. (204-371° C.) are hydrotreated at mild conditions over a fixed bed of hydrotreating catalyst (typically nickel-molybdenum on alumina) and recycled as coal slurry oil. From an Illinois No. 6 coal, liquid yields of over 40 w % on dry ash free (“DAF”) coal were obtained during the 2-year demonstration program.
Additionally, the H-Coal Process invented by Hydrocarbon Research, Inc. and is generally described in U.S. Pat. Nos. 3,519,553 and 3,791,959. The H-Coal Process uses a single ebullated-bed reactor with a hydroconversion catalyst to convert coal to liquid fuels. The ebullated-bed reactor is unique in its ability to process solids containing streams in the presence of high activity hydrogenation catalyst particles. Product oil (400° F.+) (204° C.+) was used to slurry the coal for feeding to the reactor. Coal liquefaction took place at temperatures of 800-875° F. (427-468° C.), and hydrogen partial pressures of 1500-2500 psia (103-172 bars). With Illinois No. 6 coal, liquid yields of greater than 50 w % on DAF coal were achieved during the multi-year year demonstration program at the 200 ton per day H-Coal Pilot Plant in Catlettsburg, Ky. The DCL technologies demonstrated commercial readiness, however, no commercial projects proceeded as oil prices fell and oil supplies increased.
In the 1980's and 1990's research continued at a smaller scale to improve the DCL technologies and reduce investments and operating costs. The Catalytic Two-Stage Liquefaction Process (CTSL) was invented by Hydrocarbon Research, Inc., as described in U.S. Patent Nos. 4,842,719, 4,874,506, and 4,879,021, to substantially increase the yield of distillate liquids from coal. For Illinois No. 6 bituminous coal, liquid yields were increased from 3 barrels per ton of MAF coal for the single stage H-Coal Process to about 5 barrels per ton of MAF coal for the CTSL Process. This was achieved by dissolving the coal feed at mild conditions while simultaneously hydrogenating the coal recycle solvent and coal liquids produced at temperatures from 600-800° F. (316-427° C.), hydrogen partial pressures of 1500-2500 psia (103-172 bars) in the presence of a hydrogenation catalyst.
In the CTSL Process, the unreacted coal from the initial stage is then fed to a direct-coupled second stage reactor operating at higher temperatures of approximately 800-850° F. (427-454° C.) and at similar pressures (1500-2500 psia) (103-172 bars) with a hydroconversion catalyst, to achieve maximum coal conversion and high distillate liquid yields. The-hydrogenation catalyst used for the single-stage and two-stage processes deactivates at these reactor conditions due to the deposition of coke and also soluble metals from the coal feed if present.
Unexpectedly and contrary to the prior art, it was learned that when the first stage reactor was maintained at least 25° F. lower temperature than the second stage reactor in the CTSL process, the coke deposited on the first stage catalyst is substantially lower than on the second stage catalyst. Moreover, the first stage catalyst activity was substantially higher than the second stage catalyst.
The low temperature first stage of direct coal liquefaction provides excellent hydrogenation of the recycle slurry oils while coal liquefaction takes place. The hydrogenation increases the coal liquid hydrogen content and also stabilizes the coal liquids.
As international fuel quality specifications have become more stringent, there became a growing need for converting coal to liquids having extremely low levels of contaminants (sulfur, nitrogen), low aromatics content, and high cetane indexes.
An improved multi-stage catalytic process to efficiently and economically process coal pyrolysis oils, a potentially very fouling hydrocarbon feedstock, is disclosed herein. These coal pyrolysis oils are highly aromatic, olefinic, unstable, and contain objectionable sulfur, nitrogen, and oxygen contaminants, and may contain coal solids which will plug fixed-bed reactors.
In the present text, “coal pyrolysis oils” covers non-gaseous products obtained from thermal treatments of coal, including gasification, and having characteristics here after described.
Accordingly, it is an objective of this invention to provide a method for the upgrading of coal pyrolysis oils, and advantageously other poor quality aromatic oils such as heavy cycle oil (“HCO”) and light cycle oil (“LCO”) from Fluid Catalytic Cracking, to obtain liquid hydrocarbons having substantially improved fuel value.
It is a further objective of this invention to use a back-mixed ebullated-bed reactor system to efficiently utilize the large heat of reaction.
It is yet another objective of the invention to use a lower temperature pretreatment stage to stabilize and increase the reactivity of the feedstock, reduce catalyst deactivation, and improve selectivity to desired distillate liquid products. Still another objective of the invention is to use a higher temperature second and possibly a third stage reactor to achieve the desired level of heavy oil conversion thus achieving better selectivity to high quality distillate oil products.
Another objective of the invention is to improve catalyst utilization and decrease the makeup catalyst requirements by cascading the spent catalyst from the low temperature stage 1 to the higher temperature second or third stage reactors where the remaining catalyst activity can be further utilized.
An additional objective of the invention is to recycle a portion of the heavy unconverted bottoms from the second or third stage product to blend with the raw coal liquids feedstock to minimize heat exchanger and heater fouling from the highly unstable coal pyrolysis oils when processing in the reactor system.
More specifically, the invention relates to a process of processing coal pyrolysis oils containing at least 25% wt of compounds boiling below 360° C. (680° F.) and less than 40% wt boiling at least at 520° C. (968° F.), said coal pyrolysis oil also containing less than 20% wt of coal fines, and at least 1% wt of oxygenated compounds (calculated as oxygen), said process comprising:                a) combining a hydrogen stream with said coal pyrolysis oil        b) feeding the combined stream from step a) to a first ebullated-bed reactor containing a hydrogenation catalyst to remove compounds as olefins, diolefins, and nitrogen, sulfur, and oxygen contaminates, add hydrogen and create a stabilized stream; and        c) feeding said stabilized stream to a second ebullated-bed reactor containing hydrogenation catalyst to remove additional heteroatoms and convert the stream; andwherein steps a-c result in the conversion of the 343° C.+(650F.°+) material in said coal pyrolysis oil in the range of between 10 wt % and 99 wt %.        